1. Field of the Invention
The present invention relates to the generation of commands applied to a flight director or automatic pilot system for guiding an aircraft to exit a windshear encounter, and more particularly to control the flight path of an aircraft equipped with an inertial platform system in an optimal manner.
2. Description of the Prior Art
The phenomenon of windshear can pose a serious threat to the safety of aircraft and in fact is considered directly responsible for several aircraft accidents, the most recent being the crash of an L-1011 aircraft at Dallas, Texas, on Aug. 2, 1985. Windshear can be defined as a rapidly changing horizontal wind or steady vertical wind, whose effect on the aircraft is to cause large speed or altitude deviations from normal flight. Windshear, either in itself or as a result of attempts by the human pilot to restore the aircraft to its normal flight path, can cause the aircraft to stall or crash.
In the prior art, the detection and guidance provided during a windshear encounter would cause the aircraft to fly at some fixed speed, usually slightly greater than stall speed. The speed commanded was usually a speed known as stick shaker speed, approximately five percent greater than stall speed, and is the speed where artificial means are used to vibrate the control column or stick to warn the human pilot of impending stall. Stick shaker speed is generally considered to be the minimum speed for safe flight. Corresponding to stick shaker speed is a stick shaker angle of attack which is generally considered to be the maximum allowable angle of attack for safe flight of the aircraft.
As many commercial transport aircraft, general aviation aircraft, and military aircraft are equipped with a flight director system whereby pitch command signals may be displayed to the human pilot the guidance command for windshear encounter is usually presented as a displacement of the pitch command bar. When the human pilot maneuvers the aircraft in such a manner as to reduce the displacement to null, he has assured that the aircraft is at the required pitch angle to satisfy the guidance command. In addition, many aircraft are also equipped with an automatic pilot system which can be used to manipulate the elevator control surface of an aircraft in order to respond to a predetermined guidance control law, such as one which might be used to command the aircraft to the optimum flight path in the event of a windshear encounter. Furthermore, many modern aircraft are equipped with inertial platform systems, used primarily for navigation, which are capable of measuring the flight path angle of the aircraft relative to the ground in an accurate manner. Inertial navigation is a method for determining the position and velocity of an aircraft by measuring its acceleration and processing the acceleration information in a computer. Its indications of position and velocity are substantially independent of vehicle maneuvers and completely self-contained since it is based on measurements made within the aircraft itself.
A shortcoming of the prior art is that the commanded fixed speed or angle of attack may result in the aircraft flying at the minimum safe speed when the magnitude and duration of the windshear do not in fact require such a maneuver. In addition, a command to fly at the maximum angle of attack can cause the excitation of a long period, poorly damped oscillation of the aircraft known as the phugoid mode. Excitation of this mode can result in the aircraft crashing even after the windshear condition has disappeared. Consequently, prior art systems could in fact create dangerous situations wherein the aircraft will crash even in the presence of a relatively low magnitude shear.
In pending application Ser. No. 834 729, now U.S. Pat. No. 4,797,674 assigned to the assignee of the present invention, sensed longitudinal and vertical components of windshear and angle of attack are combined to generate a command to reduce the aircraft's true airspeed at a rate proportional to the magnitude of the encountered windshear. This scheme effectively minimized the flight path angle change in a shear encounter and provided better guidance commands, but did not adequately take into account the long term phugoid mode oscillations of the aircraft.
A further prior art guidance apparatus is disclosed in applicant's pending application Ser. No. 920,402, now FWC 07/198,602, also assigned to the assignee of the present invention, wherein actual flight path angle derived from a vertical gyroscope is combined with angle of attack to generate a predetermined flight path angle. A command was generated that produced an optimal flight path angle relative to the earth, but suffered from inaccuracies in the measuring devices, specifically in the input parameters of pitch angle and angle of attack. Pitch angle is typically derived from a vertical gyroscope aboard the aircraft and is subject to well-known measurement errors in accelerated flight. Specifically, the actual pitch angle may differ by as much as two degrees from the measured value due to gyroscope erection to a false vertical caused by acceleration or deceleration of the aircraft's speed relative to the ground. In this case, the gyroscope erection circuitry senses the vector sum of the gravitational vector and the acceleration vector and missaligns the gyro along the vector direction.
In addition to the pitch angle error, errors can be introduced by the angle of attack probe. The angle of attack of an aircraft is a basic parameter of flight extremely useful in the guidance and control of an aircraft. Angle of attack is generally measured by probes or vanes protruding from the outside surface of the aircraft. Angle of attack probe errors can be caused by tolerances in alignment procedures which typically allow plus or minus 0.5 degrees. Consequently, an error of 0.5 degree can occur in the angle of attack measurement. In the worst case the flight path angle computed from pitch angle measurement minus the angle of attack measurement can produce errors in the desired flight path angle of plus or minus 2.5 degrees. These errors are sufficient to result in significantly degraded performance in a windshear encounter.
The present invention overcomes the limitations of the prior art by providing a guidance command that not only effectively minimizes the excitation of the aircraft's phugoid mode but also is not prone to the above-mentioned measurement errors. The invention will maximize the time the aircraft remains in the air and distance traveled, regardless of the magnitude of the windshear or whether the winds are horizontal, vertical or a combination of the two. By utilizing outputs from an on-board inertial platform system, angle of attack errors are eliminated and pitch angle errors are held to a minimum, thus allowing the automatic pilot or human pilot to fly the aircraft along the optimum flight path angle more accurately and easily. The state of the art in inertial platform systems is such as to provide inertial flight path angle measurements with accuracies of plus or minus 0.25 degrees.